The present invention relates to a display device provided with a plurality of unit pixels driven by thin film transistors and a method for manufacturing the display device.
In recent years, flat panel display type display devices based on an active matrix system provided with driving elements including film transistors (TFTs) have been noted. Most of all, a display device taking the advantage of organic electro luminescence (EL) has been developed toward commercialization.
Here, the principle of light emission of a unit pixel of an organic EL display device based on the active matrix system will be described with reference to an equivalent circuit of the unit pixel shown in FIG. 7. A signal line is represented by Y1, a current supply line is represented by Y2, and scanning lines are represented by X1 and X2.
A voltage is always supplied to the current supply line Y2. When a scanning pulse is applied to the scanning line X1 and a predetermined signal is supplied to the signal line Y1, a second transistor Tr2 is turned on, and a predetermined signal is written into a storage capacitor C. A first TFT transistor Tr1 is turned on based on the written signal, a current in accordance with the amount of signal is supplied through the current supply line Y2 to a light emitting portion EL and, thereby, the light emitting portion EL emits light.
A plurality of the unit pixels are arranged in the matrix, so that a display device is configured.
The configuration of each thin film transistor will be described below. FIG. 8 shows a bottom gate type known thin film transistor 101 having a channel region made of p-Si.
In the thin film transistor 101, a gate electrode 103 is disposed on a molybdenum thin film on a substrate 102. a gate insulating layer 104 made of SiN and SiO2 and a crystalline Si layer 105 are disposed over the gate electrode 103 and the periphery thereof.
The crystalline Si layer 105 is provided by irradiating an amorphous silicon (α-Si) layer with a laser like an excimer laser.
In the case where crystallization is effected with the excimer laser or the like, as shown in FIG. 9A, pulsed laser light 203 shaped to have substantially the same width as the width of a unit region 202 including a thin film transistor formation portion is stepped sequentially in a direction perpendicular to a major axis direction (width direction) of the laser light 203, and is continuously applied to amorphous silicon. In FIG. 9A, reference numeral 204 denotes a laser irradiation region.
However, the pulsed laser light 203 has energy variations of about 5% on a pulse basis. Furthermore, the laser light 203 also has an energy distribution in a minor axis direction (movement direction a). Therefore, as shown in FIG. 9B, an intense energy point (indicated by a broken line in the drawing) may occur at an energy irradiation end.
In the case where laser light exhibiting nonuniformity due to variations among pulses or in a pulse, as described above, is applied, in order to leveling the irradiation energy in the unit region 202 to be irradiated, for example, a step movement between infinitesimal sections in the minor axis direction and laser irradiation are repeated. Furthermore, 90% to 95% of regions of the individual pulses are allowed to overlap one another and, thereby, the difference in cumulative irradiation energy in a unit region 202 is reduced.
However, the crystallinity of individual unit pixels may not be satisfactorily leveled by such a technique, and luminance variations are recognized during the operation of a display device 121 (refer to FIG. 10).
For the purpose of leveling characteristic variations among TFT elements in a current mirror or a differential circuit, Japanese Unexamined Patent Application Publication No. 10-197896, for example, proposes that TFT elements in the circuit are arranged to become symmetric with respect to a point, the TFT elements located at the positions symmetric with respect to the point are connected in series, individual characteristic variations which inevitably occur within a certain range are leveled and, as a result, the characteristics of nearby two pairs of TFT series circuits are leveled.
However, according to the technique disclosed in Japanese Unexamined Patent Application Publication No. 10-197896, since there is a need for connecting the thin film transistors in series, the configuration of the driving element becomes complicated, and the yield is decreased. In addition, when the laser is applied with the step movement, as described above, the gate wiring is also be irradiated with the laser light. For example, in the case where the gate wiring is made of a material, such as aluminum, having low thermal resistance, the heat due to the laser irradiation is conducted to the gate wiring and, therefore, abnormal crystal growth, e.g., hillock or whisker, is allowed to proceed, and short-circuiting results. Therefore, there is a limit of material to be used as the gate wiring.